Periodic Reporting for period 1 - Green-Combustion (Addressing challenging issues for turbulent premixed hydrogen combustion modeling using novel technologies)
Periodo di rendicontazione: 2021-09-01 al 2023-08-31
For hydrogen combustion, the most challenging issues are the augmented effects of differential diffusion leading to thermodiffusive instabilities, which can substantially change flame dynamics and heat release rates. Although hydrogen is commonly regarded as a green fuel since it does not emit greenhouse gases, nitrogen oxides (NOx) can be formed during its combustion in the air. It was found that the formation of NOx via the thermal pathway is the primary reaction pathway at close to stoichiometric conditions due to the high temperatures. In fuel-lean premixed hydrogen flames, NOx emissions can be reduced due to the overall lower flame temperatures. However, intrinsic instabilities in premixed hydrogen flames lead to cellular structures, which directly influence local heat release and the local fuel-air ratio and lead to local super-adiabatic temperatures, which control the local reaction pathways of NOx formation. In this project, the characteristic patterns and NOx formation mechanism in the thermodiffusively unstable premixed hydrogen flame are investigated by performing large-scale direct numerical simulations (DNS).
In Green Combustion, the following research questions are addressed: (1) The characteristic patterns of thermodiffusively unstable premixed hydrogen flame in a sufficiently large computational domain are quantified; (2) The NOx formation mechanism in the thermodiffusively unstable premixed hydrogen flame is investigated through a reaction pathway analysis; (3) The effects of computational setup (2D vs. 3D) on the characteristic patterns and the NOx reaction pathways are quantified; (4) A new flamelet tabulation method is proposed to predict NOx formation in thermodiffusively unstable premixed hydrogen flames, in which the effects of curvature are considered.
The overall objectives of this project are to understand the characteristic patterns and the NOx formation mechanism in thermodiffusively unstable premixed hydrogen flames and to accurately predict the thermodiffusively unstable premixed hydrogen flame using a high-fidelity combustion model.
In Green Combustion, the following research questions are addressed: (1) The characteristic patterns of thermodiffusively unstable premixed hydrogen flame in a sufficiently large computational domain are quantified; (2) The NOx formation mechanism in the thermodiffusively unstable premixed hydrogen flame is investigated through a reaction pathway analysis; (3) The effects of computational setup (2D vs. 3D) on the characteristic patterns and the NOx reaction pathways are quantified; (4) A new flamelet tabulation method is proposed to predict NOx formation in thermodiffusively unstable premixed hydrogen flames, in which the effects of curvature are considered.
The overall objectives of this project are to understand the characteristic patterns and the NOx formation mechanism in thermodiffusively unstable premixed hydrogen flames and to accurately predict the thermodiffusively unstable premixed hydrogen flame using a high-fidelity combustion model.
The work performed in this project includes the following aspects:
(1) The NOx formation mechanism in a 2D thermodiffusively unstable premixed hydrogen flame was investigated through DNS, and the performance of the flamelet model in predicting the NOx species in the thermodiffusively unstable premixed hydrogen flame were assessed through an a priori analysis;
(2) A large-scale 3D DNS of thermodiffusively unstable premixed hydrogen flames in a sufficiently large computational domain was performed, requiring about 67 Million CPU hours;
(3) The characteristic patterns of thermodiffusively unstable premixed hydrogen flames were quantified, including the global burning velocity, flame surface area and stretch factor;
(4) The NOx formation mechanism of thermodiffusively unstable premixed hydrogen flames was investigated through a reaction pathway analysis;
(5) A flamelet tabulation method was proposed to predict NOx formation in thermodiffusively unstable premixed hydrogen flames, accounting for curvature effects.
Based on the large-scale DNS dataset, we found that the global burning velocity in the 3D computational domain is about 70% higher than the 2D simulation, which is mainly related to the increased flame surface area. In particular, the range of positive curvature in the 3D simulation is much wider than in the 2D simulation. The peak concentration and production rate of H radicals in the 3D simulation are around two and five times higher than in the 2D simulation, respectively. Over 90% NO is formed in the positively-curved regions, with the NNH reaction pathway being dominant in the 3D thermodiffusively unstable premixed hydrogen flame. The thermal-NO reaction pathway is overall negligible for both 2D and 3D simulations. Compared to the conventional flamelet model, the radicals (e.g. H, N, NH, NNH) that are sensitive to the local curvature value can be accurately predicted by the new flamelet model. However, the prediction accuracy of NOx species mass fractions and their production rates did not significantly improve compared to the conventional flamelet model, which is due to their slow chemistry and the non-uniqueness of the flamelet table.
(1) The NOx formation mechanism in a 2D thermodiffusively unstable premixed hydrogen flame was investigated through DNS, and the performance of the flamelet model in predicting the NOx species in the thermodiffusively unstable premixed hydrogen flame were assessed through an a priori analysis;
(2) A large-scale 3D DNS of thermodiffusively unstable premixed hydrogen flames in a sufficiently large computational domain was performed, requiring about 67 Million CPU hours;
(3) The characteristic patterns of thermodiffusively unstable premixed hydrogen flames were quantified, including the global burning velocity, flame surface area and stretch factor;
(4) The NOx formation mechanism of thermodiffusively unstable premixed hydrogen flames was investigated through a reaction pathway analysis;
(5) A flamelet tabulation method was proposed to predict NOx formation in thermodiffusively unstable premixed hydrogen flames, accounting for curvature effects.
Based on the large-scale DNS dataset, we found that the global burning velocity in the 3D computational domain is about 70% higher than the 2D simulation, which is mainly related to the increased flame surface area. In particular, the range of positive curvature in the 3D simulation is much wider than in the 2D simulation. The peak concentration and production rate of H radicals in the 3D simulation are around two and five times higher than in the 2D simulation, respectively. Over 90% NO is formed in the positively-curved regions, with the NNH reaction pathway being dominant in the 3D thermodiffusively unstable premixed hydrogen flame. The thermal-NO reaction pathway is overall negligible for both 2D and 3D simulations. Compared to the conventional flamelet model, the radicals (e.g. H, N, NH, NNH) that are sensitive to the local curvature value can be accurately predicted by the new flamelet model. However, the prediction accuracy of NOx species mass fractions and their production rates did not significantly improve compared to the conventional flamelet model, which is due to their slow chemistry and the non-uniqueness of the flamelet table.
State-of-the-art 3D DNS of a thermodiffusively unstable premixed hydrogen flame was performed in a sufficiently large computational domain, excluding the effects of confinement on the global burning velocity are excluded. Based on the 3D DNS dataset, the characteristic patterns of the thermodiffusively unstable premixed hydrogen flame were quantified for the first time, and a new flamelet model was proposed to predict NOx formation in thermodiffusively unstable premixed hydrogen flames, accounting for curvature effects by solving the flamelet equations in composition space.
Hydrogenate focused on key open challenges in hydrogen combustion using an approach combining high-fidelity simulations and the development of engineering models to simulate systems of practical interest, aiming to provide tools to make hydrogen combustion safe and clean regarding pollutant formation. In particular, in the MSCA project, the following milestones were accomplished: (1) Direct Numerical Simulations of the thermodiffusively unstable premixed hydrogen flames in 2D and 3D computational domains, accounting for NOx formation. (2) Development of a flamelet tabulation method for modelling NOx in thermodiffusively unstable premixed hydrogen flames, accounting for curvature effects.
The research has the potential to strongly impact the academic community and society as a whole. Indeed, hydrogen combustion will play a key role in the future energy scenario as a replacement energy carrier for fossil fuels. In this context, understanding the NOx formation mechanism in hydrogen flames is important since pollutant reduction techniques can be proposed and tailored relying on the specific properties of hydrogen and the findings of the present project. For example, we demonstrated that the NOx emissions in the thermodiffusively unstable premixed hydrogen flames are increased by about five times compared to the planar flames. Thus, the thermodiffusive instabilities should be minimised in practical applications (e.g. by increasing the fuel temperature, the fuel equivalence ratio, etc…).
Hydrogenate focused on key open challenges in hydrogen combustion using an approach combining high-fidelity simulations and the development of engineering models to simulate systems of practical interest, aiming to provide tools to make hydrogen combustion safe and clean regarding pollutant formation. In particular, in the MSCA project, the following milestones were accomplished: (1) Direct Numerical Simulations of the thermodiffusively unstable premixed hydrogen flames in 2D and 3D computational domains, accounting for NOx formation. (2) Development of a flamelet tabulation method for modelling NOx in thermodiffusively unstable premixed hydrogen flames, accounting for curvature effects.
The research has the potential to strongly impact the academic community and society as a whole. Indeed, hydrogen combustion will play a key role in the future energy scenario as a replacement energy carrier for fossil fuels. In this context, understanding the NOx formation mechanism in hydrogen flames is important since pollutant reduction techniques can be proposed and tailored relying on the specific properties of hydrogen and the findings of the present project. For example, we demonstrated that the NOx emissions in the thermodiffusively unstable premixed hydrogen flames are increased by about five times compared to the planar flames. Thus, the thermodiffusive instabilities should be minimised in practical applications (e.g. by increasing the fuel temperature, the fuel equivalence ratio, etc…).